1. Field of the Invention
The present invention relates to a microscopic grinding method and to a microscopic grinding device, and more particularly, to a method of precision grinding a minute area to achieve ultrahigh precision processing in such fields as the manufacture of lenses and other optical devices, and to a polishing unit used in said method.
2. Description of the Prior Art
X-ray optical elements and aspherical lenses used in a variety of electronics and optical devices are manufactured with grinding and polishing processes, but these processes typically require microscopic grinding precision to achieve shapes precise to a maximum 0.01 .mu.m. This requires methods enabling extremely high precision grinding within an extremely minute area.
Polishing and lapping are two widely used conventional high precision grinding methods, but these methods cannot achieve ultrahigh precision grinding on the order of a maximum 0.01 .mu.m. Magnetic grinding methods using a magnetic fluid or magnetic grinding agent have gained attention in recent years as methods of achieving higher precision than is possible with the aforementioned methods.
A magnetic single body or magnetic grinding fluid with a granular grinding agent suspension dispersed in a magnetic fluid are used in magnetic grinding methods. This magnetic grinding fluid is supplied between the tip of the grinding tool and the workpiece and a magnetic field is applied between the grinding tool and workpiece to hold the magnetic grinding fluid between the grinding tool and workpiece. The magnetic grinding fluid is thus held by this magnetic action in a state with pressure applied to the grinding surface of the workpiece. If in this state the grinding tool is rotated at high speed, the magnetic grinding fluid is pulled and moved by the rotation of the grinding tool, causing the grinding agent grains suspended in the magnetic grinding fluid to collide with the workpiece, and thereby grinding the workpiece surface. By varying the direction and strength of the magnetic field, the applied pressure of the magnetic grinding fluid on the grinding surface can be adjusted, the action of the grinding agent grains suspended in the magnetic grinding fluid can be controlled, and grinding performance can be improved.
A grinding device or method for grinding a workpiece with the use of magnetic grinding fluid, for example, Japanese Laid-open Patent Publication No. 60-118466 published June 25, 1985, Japanese Patent Publication (examined) No. 1-16623 published Mar. 27, 1989 or Japanese Patent Publication (examined) No. 1-16623, published Mar. 27, 1989, is disclosed.
According to Japanese Laid-open Patent Publication No. 60-118466, the magnetic grinding fluid is gathered by a magnetic field which is generated by an array of magnets arranged such that the magnetic pole of any two adjacent magnets are opposite to each other.
According to Japanese Laid-open Patent Publication No. 61-244457, the rotating magnetic field is formed between the grinding tool and the workpiece.
According to Japanese Patent Publication (examined) No. 1-16623, the workpiece is dipped into the grinding fluid and placed between the rotating lapping tool and an opposed iron core where a static magnetic field is formed to tract and concentrate the magnetic grinding fluid to a gap formed between the workpiece and the rotating lapping tool.
Because the action of the grinding agent can be focused by the magnetic retention force on a minute region of the workpiece using this magnetic grinding method, grinding and polishing with higher precision than is possible with conventional grinding methods can be achieved.
However, even magnetic grinding methods as described above cannot achieve ultrahigh precision grinding on the order of a maximum 0.01 .mu.m. In addition, this method is inapplicable with workpieces made of a non-magnetic material.
While the magnetic grinding fluid suspended between the tip of the grinding tool and the workpiece grinding surface is moved by the high speed rotation of the grinding tool and thus grinds the workpiece surface, unavoidable fluctuations in rotational speed and axial vibrations caused by rotation of the grinding tool result in variations in the grinding depth, uneven grinding with localized variations within the grinding area, and fluctuations in the grinding area in the finish of the grinding surface, which is directly affected by the rotation of the grinding tool. Sufficient allowance must be provided in the mechanical drive mechanism to allow the rotating members to move smoothly. This allowance necessarily produces slight gaps and play which make it impossible to completely prevent variations and unevenness in the finish of the grinding surface insofar as the grinding tool is rotated at high speed.
Furthermore, the applied pressure which presses the magnetic grinding fluid against the grinding surface to produce a grinding action results from the application of a magnetic field. Because this pressure is not generated by the rotation of the grinding tool, the applied magnetic field strength must be increased to obtain a sufficient grinding action. In addition, because variations in the field strength also affect the grinding depth, the field strength must be strictly controlled, and the electromagnet or other magnetic field generating means accordingly becomes larger.
In addition, with a conventional magnetic grinding method, the magnetic grinding fluid is held between the grinding tool and the workpiece by passing magnetism from the grinding tool to the workpiece, and the workpiece itself is thus a component of the magnetic circuit. As a result, the workpiece must be made of a magnetically conductive, i.e., a magnetic material. It is to be noted that if the workpiece is a non-magnetic material, the magnetic circuit can still be formed through the non-magnetic workpiece if the workpiece is thin enough. Lenses and other optical elements, however, are both non-magnetic and typically fairly thick, and cannot therefore be processing with this conventional magnetic grinding method. Moreover, because the magnetic action of the workpiece on the magnetic grinding fluid is also dependent upon the thickness and magnetic properties of the workpiece, the workpiece also has a significant affect on the pressure applied from the magnetic grinding fluid on the workpiece surface and on the grinding finish, making it difficult to precisely control the grinding depth and grinding precision.